This invention relates to the determination of concentrations of carbon monoxide in general and more particularly to an improved method and apparatus for making such a determination.
There is a requirement for measuring the concentration of carbon monoxide in many different areas. For example carbon monoxide emissions from industrial plants, motor vehicles and domestic heating plants must be measured in many instances. With recent emphasis on occupational safety the need for measurement and recording of the carbon monoxide content of the air resulting from these types of emissions has been recognized.
For the purpose of measuring carbon monoxide in emissions from various sources and for measuring the carbon monoxide level in a particular area various measurement principles have been used. In particular methods utilizing a direct chemical reaction, methods including a combustion chamber and measurement of the conductivity of a reaction solution, methods using gas chromatography, methods using measurement of the heat change and methods using measurement of infrared absorption have been used (see VDI-Berichte No. 180: Carbon monoxide - generation, measurement and criteria of effects, VDI-Verlag GmbH, Duesseldorf, 1972, page 6 to 9). These known methods of measuring CO concentrations are not completely satisfactory. In particular they are not well suited for measurements in an area where personnel are exposed to concentrations of carbon monoxide. Furthermore in general they exhibit an appreciable sensitivity to other gases (cross sensitivity), so that interfering gases must be removed from the test sample prior to measurement. In addition the lowest detection limit in commercially available equipment which is suitable for automated anaylsis is about 10 ppm. (1 ppm corresponds to 0.0001% by volume).
However, in an area in which personnel are exposed to concentrations of carbon monoxide it is necessary that concentrations near and below 10 ppm be measured exactly. In many countries exposure levels have been established. For example in the Federal Republic of Germany the following Maximum Immission Concentration values were approved as early as 1967: 8 ppm of CO for 24 hours, 16 ppm of CO for 8 hours and 80 ppm of CO for 1 hour. In the U.S. even lower values have been established: 9 ppm of CO for 8 hours and 35 ppm of CO for 1 hour. By comparison the CO content of natural air, away from all civilization, is 0.012 ppm. Thus, the range of concentrations of carbon monoxide which must be measured extends far below 50 ppm which in Germany is the maximum permissible work station concentration.
In view of these recent requirements for maintaining low and safe levels of carbon monoxide in work areas the need for a method which is simple and can be automatically carried out and which can be used for measuring both emissions from polluting devices as well as measuring concentrations to which personnel are exposed is evident. Furthermore, such a device which exhibits little or no cross sensitivity and which permits determining low CO concentrations accurately, i.e., concentrations between 0 and 50 ppm is required.